Title: The Non-Uniform Expansion of the Crab Nebula
Authors: T. Martin, D. Milisavljevic, T. Temim, S. Mandal, P. Duffell, L. Drissen, Z. Ding,
First Author’s Institution: Département de physique, de génie physique et d’optique, Université Laval, Québec
Status: Preprint hosted on the ArXiv
Massive stars burn bright and die young, in their final moments exploding in a spectacular supernova. Most of the stuff that once made up the star is then expelled out into the interstellar medium in what are called supernova ejecta. For some time after the initial explosion, we can still see these ejecta making their way out into the vast unknown; this material remains bright, powered by a super hot temperature, radioactive decay and even radiation from the ‘dead star’ that sometimes lies at the centre of the supernova remnant nebula.
The famous Crab Nebula is one such supernova remnant. The nebula spawned from a supernova explosion that was historically observed by Chinese astronomers in approximately 1054 CE, making it one of the only Galactic supernova events in our recorded history. Having such a well defined date of explosion makes astronomer’s jobs more interesting; we are able to use this information to better constrain our models of supernovae and how their ejecta behave over time.
Today’s authors ask specifically how the Crab has changed over our human timescales and what this tells us about its distant past. To do this, they imaged the entire nebula using the same instrument on board the Canada-France-Hawaii Telescope (CFHT) over a time span of 12 years from 2007 to 2019 – about ~1% of the nebula’s total age! Using the same telescope with the same settings allows easier comparison of the nebula over time and removes a lot of room for error. The two images, coloured by their year of imaging, are shown stacked in Figure 1.
Figure 1: Two of the three Crab Nebula images analysed in the study are shown here stacked on top of each other. The 2007 observation is coloured in red with the 2016 observation coloured in blue. Three zoomed regions are shown in the right column, showing the obvious expansion across the years in regions of interest via the separation of colours. Source: Figure 1 in the paper.
What’s clear immediately from comparing the images is that the nebula has visibly expanded, even after as little as 12 years! In particular, the difference in nebula ridge position appears to be greater towards the periphery of the nebula due to projection effects of flattening a 3D nebula down to what we see in the sky in 2D. That is, filaments far from the nebula centre seem to be expanding faster. Figure 2 shows that this holds for the vast majority of features within the Crab Nebula.
By comparing the expansion vectors of various filaments and features, together with their radial position from the nebula centre, the authors can extrapolate backwards to estimate at what year the explosion took place. We should, of course, expect this date to line up with the historical date of 1054 CE. When the authors do this, however, they find an average origin date of about 1105.5 CE – a full 50 years discrepant. Where models disagree with the data, there is new physics to be found!
Figure 2: About half of the nebula filament expansion vectors are shown here overlaid on the 2019 image of the Crab. The magnitude and direction of each vector shows their predicted position 50 years from the paper publication date (so approximately the year 2075). The red cross near the centre marks the estimated central point of the nebular expansion. Source: Figure 5 in the paper.
The resolution for this date discrepancy lies in the fact that the supernova explosion all those years ago left a fossil at the heart of the nebula: a rapidly rotating neutron star christened the Crab Pulsar. This pulsar is known to harbour an intense wind that would have accelerated the nebular material for some time after the supernova explosion took place. When taking into account the acceleration of material due to the wind, together with the fact that the wind weakens over time due to the spin-down of the pulsar, the authors are able to accurately model the expansion properties of the nebula and arrive at an origin date consistent with historical records.
As part of the paper, the authors made an estimate of the central nebula position (Figure 2, red cross) which can be used to compare to the apparent position of the Crab Pulsar; using this together with the apparent motion of the neutron star can inform of the strength of any supernova kick that may have occurred. There is also evidence that the bulk expansion velocity is not spherically uniform around the projected nebula, hinting at a disk of pre-existing circumstellar material or an asymmetric supernova explosion.
Studies such as these are essential in quantifying our understanding of supernovae. It is with these analyses of expansion kinematics that astronomers can work their way backwards to the scene of the supernova crime, to better understand what happens in a star’s final moments and the dynamics of their fossil remains throughout the Galaxy.
Astrobite edited by Chloe Klare
Featured image credit: Martin et al 2025, Dominic Alves
Author
I am a masters student at Macquarie University in Australia, working mainly on binary/multiple systems with massive stars (Wolf-Rayets in particular!). Outside of study, I’m a novice film buff, baking sourdough all the time, probably drinking coffee, and trying to get more into reading and frisbee/squash. You can also find me procrastinating on bluesky @astroryan.bsky.social
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